Radiotherapeutic apparatus is well-known, and consists of a source of radiation which emits a beam of radiation that is directed toward a patient in order to destroy or otherwise harm cancerous cells within the patient. Usually, the beam is collimated in order to limit its spatial extent to a desired region within the patient, such as the tumour or a sub-section of the tumour. The source can be a linear accelerator for high-energy (MV) x-radiation, or an isotopic source such as Co-60.
The source is often rotated around the patient in order to irradiate the desired region from a number of different directions, thereby reducing the dose applied to healthy tissue lying around the desired region. The shape of the desired region can be changed dynamically as the source rotates, in order to build up a complex dose distribution for tumours with more challenging shapes and/or which are located near to sensitive areas.
As the dose distribution becomes more closely tied to the exact shape of the tumour, and as the accuracy of the dose delivery improves, it has become necessary to know the current position of the patient, their internal organs, and the tumour with greater accuracy. As a result, low-energy x-ray sources are often provided on the apparatus in addition to the high-energy therapeutic source, to allow for x-ray or CT imaging of the patient before or during treatment. Portal imagers are often provided, which detect the therapeutic beam after attenuation by the patient. Both provide a degree of information as to the patient, but are subject to the inherent limitations of x-ray imaging, in particular the poor contrast obtained in areas of soft tissue. Generally, x-ray imaging is able to provide good contrast between areas of bone, soft tissue, and air, which allows for the detection of the gross patient position but has difficulty in detecting internal movements of the patient and the sub-structure within the soft tissue.
Efforts have therefore been directed towards combining a radiotherapy source with an MRI imager. MRI provides contrast within soft tissue, and is therefore suitable. However, there are significant practical problems in combining these two very different technologies.